In a typical VAD process soot preforms are prepared by reacting glass precursors in an oxyhydrogen flame to produce silica particles. The silica particles are deposited on a starting rod. The starting rod is slowly pulled upward while it is rotated, and the silica particles are deposited axially on the rod as it is pulled. Very large, and long, soot preforms can be continuously fabricated. Typically the soot for the core is produced by a core torch and the soot for the cladding by a cladding torch. In this way, the composition of the glass can be varied from the center portion of the preform to the outside portion. Variation in glass composition is required for providing the refractive index difference necessary to produce light guiding in the optical fiber. After the soot is deposited, the preform is heated to consolidate the silica particles into a solid transparent glass body. Optical fiber is manufactured by drawing fiber from the consolidated preform using a conventional fiber drawing apparatus.
A variety of refractive index profiles are used in commercial optical fiber. These vary from simple step index profiles to graded index profiles to profiles with multiple index values. In most cases the center core of the preform used to draw these different varieties of fibers has a center core that is the same in each case.
For preforms made by a VAD process, a separate core deposition torch is used to deposit soot for this core, and another torch, a cladding torch, is used to deposit the soot for the outer cladding portion of the preform. In some cases more than two torches are used to form multiple layers for more complex profiles. After the entire preform is deposited, it is consolidated and drawn into an optical fiber.
The VAD process is very effective and widely used for preform manufacture. However, some optical fiber profiles require a relatively sharp interface between deposited layers. In some of these designs the sharp interface is used to control bending losses. Other optical fiber profiles have an undoped (pure silica) core. And in essentially all preform designs, control over preform diameter is important. The conventional VAD process is not ideally suited for producing sharp index gradients, or preforms with undoped cores. It also does not provide sharp control over preform diameter. This is due to heat from the second (or third) torch, which causes diffusion of dopants previously deposited by the first (or second) torch radially out from the center, or causes diffusion of dopants deposited by the second or subsequent torches radially in toward the center.
These problems have been overcome in part by the rod-in-tube process, wherein a core rod is first produced and cladding is provided after the core rod is consolidated. This reduces the opportunity for diffusion of dopants between doped layers in the preform. However, the interface between the previously prepared core rod and the cladding layer often has contaminants that deteriorate the optical transmission performance of the overall preform and resulting fibers.
A variation of the rod-in-tube approach is known in which the core rod is first prepared, as in the aforementioned rod-in-tube process, then the cladding is formed by soot deposition on the core rod. However, in this method also, it is difficult to control the quality of the interface between the core rod and the deposited soot.
In both methods just described, where a core rod is produced first, it is typical to produce a core rod that consists of a center core and an inner cladding layer. Then an outer cladding layer is provided by either an overcladding tube, or deposited soot. In either case, if the core rod/primary cladding is prepared by a VAD process, the problem of diffusion between dopants in the core and the first cladding layer remains.